Headmaster reported belief (9)
I believe the answer is:
principle
'belief' is the definition.
(thesaurus)
'headmaster reported' is the wordplay.
'headmaster' becomes 'principal' (I've seen this before).
'reported' shows a homophone (sound like).
'principal' sounds like 'PRINCIPLE'.
(Other definitions for principle that I've seen before include "honour" , "Axiom" , "In which, theoretically" , "Morally correct behaviour" , "Precept, rule" .)